Such devices can be used, for example, to detect when a pressure becomes too low, as in the case of the oil pressure in the oil pump of a motor vehicle, or else to detect when a pressure becomes too high, as in an enclosure containing gas under high pressure. A warning signal is then essential to avoid the dangerous consequences that ensue from such pressure variations beyond given predetermined thresholds.
The state of the art in general is illustrated in U.S. Pat. No. 3,064,094, British Patent No. 1 304 085, and in French Patent No. 2 521 341. The following patents may also be mentioned: British Patent No. 1 440 756, U.S. Pat. No. 3,121,145, and French Patent No. 2 107 788.
Most of these prior devices include only one moving contact member, whereas in some applications it is advantageous to have two thresholds available, for example a first threshold which constitutes an early warning and a second threshold which is an alarm indicating that immediate action must be taken, which generally means that a machine must be switched off.
The present invention relates to a development of such a two-threshold device, and in particular of a two-threshold changeover pressure switch.
The term "changeover" is used in the present specification to indicate that a low pressure circuit is on when the device is subjected to a low extreme pressure, that a high pressure circuit is on when the device is subjected to a high extreme pressure, and that neither circuit is on when the device is subjected to an in-between pressure. This is in contrast to a device in which both circuits would be on at one extreme pressure, in which only one of the circuits would be on in the middle range, and in which both circuits would be off at the opposite extreme.
Such two-threshold changeover pressure switches already exist, and a typical example is described in French Patent No. 2 513 313. The device shown in this prior French patent comprises an insulating housing including contact tabs and having a cavity receiving a moving insulating member supporting a first contact member, a fixed conductive washer, and an insulating pusher passing through said washer, said pusher supporting a second contact member and coming into abutment against a membrane whose opposite face is subjected to the action of a pressure, said cavity further including resiliently deformable means tending to maintain the contact members against their supports, said contact members co-operating with said conductive washer to open and close corresponding electric circuits including corresponding ones of said contact tabs, said circuits being opened and closed as a function of the value of the pressure acting on the membrane relative to predetermined threshold pressures.
The structure of this device is complicated, in particular concerning the fixed conductive washer which is drum-shaped, having a bottom with a plurality of openings therethrough, and also concerning the associated three-rod contact member. The component parts are disposed on either side of the bottom of the drum, thereby complicating structure ad operation. In addition, reliability and accuracy suffer from the complexity of the structure. In addition to the risk of the telescopic parts jamming against each other, there is a risk of the contact disposed between the bottom of the moving insulating member and the bottom of the fixed conductive drum being crushed in the event of a sudden surge in pressure (hammering). Finally, the working section of the membrane varies during operation (pusher, then pusher plus moving member), and this requires a suitably strong spring to be provided, to the detriment of accuracy. However, in automobile manufacture in particular, it is advantageous to have monitoring and/or warning devices, e.g. for the hydraulic circuits used for lubrication or for braking, which are reliable, accurate, and competitively priced.
Preferred implementations of the present invention provide a two-threshold changover pressure switch for use in monitoring an/or providing warnings relating to abnormal pressure variations, with the switch being simple in structure, and being reliable and accurate in the long term.
Preferred embodiments of the invention also provide a pressure switch which stands up well to sudden pressure surges in the event that excess pressures are monitored.
Preferred embodiments of the invention provide a pressure switch capable of providing an immediate replacement for an existing device, without requiring complicated disassembly.